DE-A-198 37 257 discloses as a filter element a filter module which is simplified with respect to its structure and which enables production of a stage filter. Each layer of the filter medium as the filter unit has of a bed of deep-bed filter material. These beds alternate with spacers, having individual draining beds stacked together with the filter beds without gaps horizontally on top of one another transversely to the longitudinal axis of the filter module. The draining beds are sealed on alternate sides to the filtrate/unfiltered material chamber. With the known approach, any arrangement of filter beds is attainable.
By using beds both for the spacers and also for the filter layers, the known approach makes it possible to stack all beds horizontally without the need for separate support elements for example in the form of support tubes here, as is generally the case for conventional filter elements. Moreover, it is possible to stack any filter beds with different filter fineness in any sequence.
In practice, the known filter element can fail especially at high pressures of the fluid which is to be filtered, for example, because the deep-bed filter material beds cannot be adequately supported on the spacers in the form of draining beds. The latter can lead to failure of the entire filter element.
EP-B-0 817 668 discloses a filter device which preferably is designed as a bypass flow filter assembly and is used specifically for filtration of hydraulic oil supplies with a motor-driven pump and a permanently mounted filter housing which may be closed with a cover. The filter housing has an interchangeable filter element through which flow can take place from the inside to the outside and which has two cylindrical filter units arranged concentrically to its longitudinal axis. The filter units have a radial distance between themselves, and are held on the end side at least by different cover parts. The unfiltered material introduced between the radial spacing of the two filter units is routed in both directions through the respective filter unit, and is filtered in such a manner. Accordingly the filter units, with the formation of the generic prior art, are enclosed to the inside and to the outside by a support element in the form of a support tube facing the respective clean side of the known filter element. If the filter element is removed from the filter housing over the cover part of the known design, the filter housing forms a holding vessel for the dirty oil when the filter element is changed. No fouled fluid remains within the filter device. This arrangement would lead to dirt in the hydraulic circuit when the filter device is restarted. Although with respect to the inserted support elements in the form of support tubes, adequate and secure support of the actual filter units and of the filter material takes place such that failure of the filter element is precluded. The cylindrical support tubes on the inner and outer peripheral sides have only smooth guide surfaces between which passage points for the fluid are formed. Consequently, flow through the known support tubes takes place essentially transversely to their longitudinal alignment so that in the process fluid guidance is restricted. The known approach cannot be used for special filtration tasks, for example, if the intention is to eliminate fluid from the filter units with the filter material by drainage.
DE-A-44 30 341 discloses a generic filter element for filtration of fluid, with a hollow-cylindrical support element for a cup-shaped fluid filter which is closed with a removable cover, especially for oil or fuel. A filter housing receives an interchangeable filter unit. The filter unit has a length of filter material folded in a star-shape with end disks mounted on two filter material end sides to form a seal. The end disk is cover-shaped in the installation state of the filter unit forming a fluid opening. The pertinent support element prevents collapse of the filter unit as the fluid stream which is to be filtered flows through it in one direction. With this known approach, it is possible to leave the support element in the filter housing each time the filter unit is changed, so that it is not necessary to dispose of the support element which is not subject to wear together with the filter unit.
In order to achieve improved fluid guidance between the support element and the filter unit which encloses it, in one especially preferred embodiment provision is made such that the support projections are routed around the tubular support element in the manner of a 4-lead spiral along the outside periphery in order to repeatedly deflect the fluid stream around the longitudinal axis of the filter element by 360°. In this case, it is characteristic for the known embodiment of the support element that the support projections have sections with different pitches. In this way, a parallel arrangement of a total of four intermediate spaces with the shape of helical lines may be formed between the indicated support projections. These intermediate spaces are available for directed fluid flow around the outside periphery of the central part as far as the fluid overflow openings provided in its upper part. Because the support projections which form the channels extend along a helical path, continuous fluid guidance along the tubular support element is prevented. As a result of the aforementioned repeated deflection of the fluid flow, an unfavorable fluid flow characteristic occurs between the support element and the filter unit which encloses it and which is supported on it.
A comparable arrangement is disclosed according to one embodiment as shown in FIG. 3 of WO 01/91881. In this known approach, only a helical channel guide which encompasses the support element is used for fluid transport and for supporting the filter unit within the filter housing. Repeated deflection of the fluid flow around the longitudinal axis of the filter element is unfavorable in terms of energy expenditure. The support projections of the support element which form the helical channel form only a small support surface for the filter unit which is to be supported, so that in this respect reliable support, especially for larger fluid difference pressures, does not take place.
Furthermore, JP-20011293312 A discloses a filter device with a filter unit which is located in a filter housing and in which the support element is provided with a plurality of flow channels located on its outer peripheral side. The channels run in a straight line, and are used for fluid transport within the filter device when oriented to the longitudinal axis of the filter element. In spite of the good support for the filter unit which can be achieved, filtration performance is adversely affected as a result of the known linear fluid guidance.